M..M ...it XXVIII.] DISTRIBUTION OF HEAT IN THE SPECTRUM. 335 



are marked a, /5, 7, They were impressed on daguerre- 

 otype plates by resorting to the well-known processes 

 for obtaining photographs of the less refrangible regions 

 of the spectrum. The paper is republished in the pres- 

 ent volume, Memoir III. 



In view of the preceding statement and others that 

 might be given, it may, I think, be affirmed that the 

 general opinion held at the present day as to the consti- 

 tution of the spectrum is this, that there exists a heat 

 spectrum in the less refrangible regions, a light spectrum 

 in the intermediate, and a spectrum producing chemical 

 action in the more refrangible regions. An experimental 

 attempt to correct this view, and to introduce a more ac- 

 curate interpretation, will not be without interest, espe- 

 cially as it is necessarily and directly connected with the 

 important subject of photometry. In this Memoir I shall 

 offer some experiments and suggestions respecting the 

 heat of the spectrum, and in another, shortly to be pub- 

 lished, shall consider the distribution of the so-called 

 chemical rays. Among the numerous problems of actiuo- 

 chemistry, there are none more important than these. 



All the experiments hitherto made on the heat of the 

 spectrum have been conducted on the principle of ex- 

 posing a thermometer in the differently colored spaces. 

 Such was Sir W. Herschel's method. Leslie used a dif- 

 ferential with small bulbs. Melloni, Muller, Tyndall, a 

 thermo-electric pile, the form preferred being the linear. 

 This was advanced successively through all the radia- 

 tions, and the deflections of the multiplier noted. 



Is not this method essentially defective ? Does it not 

 necessarily lead to incorrect results? 



" There is an inherent defect in the prismatic spectrum 

 a defect originating in the very cause which gives rise 

 to that spectrum itself unequal refrangibility. Of two 

 groups of rays compared together, one taken in the red, 



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